This is a study of the nature and development of individual differences in certain linguistic abilities that have been found to be of significance in the learning of foreign languages, and that also are likely to be important in the learning and use of the native language, particularly in reading and writing. These abilities are thought to represent various types of knowledge and awareness of aspects of the native language system--phonological, grammatical, lexical, and semantic. In particular, phonetic coding ability, grammatical sensitivity, and naming ability deserve attention. They will be studied intensively by both experimental and psychometric techniques in order to gain better understanding of their nature and importance in language behavior. Language performances will be studied both in the experimental laboratory and in school testing situations, with samples of children of various ages (6-year-olds and up) as well as young adults. After individual differences in these linguistic ability variables are more sharply defind, the extent to which they are modifiable through training, and the extent to which they can be taken into account in teaching language skills, will be studied. A longitudinal component will trace these abilities over several years, with the primary aim of determining whether certain linguistic abilities that may be critical in learning to read are the same abilities that have been identified as critical in foreign language learning at a later period. In the experimental phases of the study, an attempt will be made to relate these abilities to parameters of certain linguistic and memory performances.